The present invention relates in general to communication systems, and in particular to methods and systems for improving various aspects of communication systems utilizing multi-carrier transmission techniques such as orthogonal frequency division multiplexing.
Wireless personal communication devices have proliferated over the past several years. Integration of more functionality such as multimedia capabilities into these devices has created an ever increasing demand for enhanced broadband communication methodologies. Unlike satellite communication where there is a single direct path from a transmitter to a receiver, personal wireless communication devices must operate in a multi-path environment. Multi-path propagation is caused by the transmitted signal reflecting off of objects such as buildings, automobiles, trees, etc., that may be encountered along the signal path. This results in the receiver receiving multiple copies of the transmitted signal each having different delay, attenuation and phase shift depending on the length of the path and the material composition of the objects along the path. The interference between the multiple versions of the transmit signal, referred to as inter-symbol interference (ISI), is a common problem that can severely distort the received signal.
Orthogonal frequency division multiplexing (OFDM) is one type of multi-carrier data transmission technique that has had some success in addressing ISI, distortion and other problems associated with multi-path environments. OFDM divides the available spectrum into multiple carriers, each one being modulated by a low rate data stream. Multiple user access is achieved by subdividing the available bandwidth into multiple channels, that are then allocated to users. The orthogonality of the carriers refers to the fact that each carrier has an integer number of cycles over a symbol period. Due to this, the spectrum of each carrier has a zero at the center frequency of each of the other carriers in the system. This results in no interference between the carriers, allowing them to be spaced as close as theoretically possible. Each carrier in an OFDM signal has a very narrow bandwidth, thus the resulting symbol rate is low. This results in the signal having a high tolerance to multi-path delay spread, as the delay spread must be very long to cause significant inter-symbol interference. Coded orthogonal frequency division multiplexing (COFDM) is the same as OFDM except that forward error correction is applied to the signal before transmission. This is to overcome errors in the transmission due to lost carriers from frequency selective fading, channel noise and other propagation effects. In the description presented herein, the terms OFDM and COFDM are used interchangeably.
In OFDM the sub-carrier pulse used for transmission is chosen to be rectangular. This allows the task of pulse forming and modulation to be performed by an inverse discrete Fourier transform (IDFT). IDFT is implemented very efficiently as an inverse fast Fourier transform (IFFT) which would then require only an FFT at the receiver end to reverse the process. In addition to the FFT, the receiver must perform channel equalization to compensate for the channel transfer function. In OFDM, channel equalization is typically performed in frequency domain to enable the estimation of channel frequency response. Several methods have been proposed for channel estimation and equalization for use with OFDM. One method transmits a known sequence (e.g., all “1's”) and any deviations from the expected received data is attributed to the channel response. This method, however, is quite susceptible to noise and yields varying channel estimates due to noise. It therefore requires additional circuitry for noise carrier suppression. Another method uses conventional least mean square algorithms and additional tuning coefficients to speed up the manipulation of the equalizer. The implementation of such equalizer is however quite complex and hardware intensive.